Examples of currently-used apparatus of the above type include an inkjet printing apparatus discharging ink droplets to print images on web paper.
Such an inkjet printing apparatus includes an inkjet head having a plurality of nozzles arranged in a width direction of the web paper. The inkjet head discharges the ink droplets, whereby the images are formed. However, the nozzles have a variation in ink discharge characteristic. Accordingly, shading correction is typically made for correcting the variation of the characteristic. Specifically, a correcting chart with a given density is printed. Thereafter, the printed correcting chart is optically read for determining densities of portions of the web paper corresponding to the nozzles individually. Then difference between the densities and the given density is calculated, and the nozzle is each corrected in accordance with the difference. This achieves a printing result having the same density for all the nozzles. See, for example, Japanese Unexamined Patent Publication No. 2014-27527A.
Note that an undulation phenomenon called cockling occurs on a surface of web paper upon transportation. The cockling is caused by swelling of a sheet due to ink droplet discharge or by dry treatment. This may result in deformation on the paper. The deformation of the web paper causes influences on optical reading of the correcting chart. Consequently, a shading difference in density occurs among the portions of the web paper corresponding to the nozzles depending on the deformation. If shading correction is performed based on density values determined under such a condition, this results in miss-correction to perform improper correction. Experience shows that the deformation occurs in a certain correlation with a printing condition such as present or absence of previous application to the web paper or a paper thickness. Accordingly, when an operator determines that the printing condition is likely to induce deformation, he/she changes a processing condition upon the shading correction for eliminating adverse effects caused by the deformation.
However, only the printing condition is not sufficient for determining whether or not the deformation occurs. Then, a device for detecting occurrence of the deformation has been suggested. See, for example, Japanese Unexamined Patent Publication No. 2001-41726A. The device detects the deformation optically while web paper is wound.
However, the example of the currently-used device with such a construction has the following drawback.
That is, the currently-used device determines deformation while the web paper is wound. As a result, the device is not applicable to a printing apparatus, for example, when deformation occurs upon transportation. Consequently, the device cannot detect occurrence of the deformation on the web paper. Such a drawback may arise.